An antenna as described above is known from published UK Patent Application No. GB 2292638A which discloses a quadrifilar antenna having an antenna element structure with four helical antenna elements formed as metallic conductor tracks on the cylindrical outer surface of a cylindrical ceramic core. The core has an axial passage with an inner metallic lining and the passage houses an axial feeder conductor, the inner conductor and the lining forming a coaxial feeder structure for connecting a feed line to the helical antenna elements via radial conductors formed on the end of the core opposite the feed line. The other ends of the antenna elements are connected to a common virtual ground conductor in the form of a plated sleeve surrounding a proximal end portion of the core and connected to the outer conductor of the coaxial feeder formed by the lining of the axial passage. The sleeve, in conjunction with the feeder structure forms a trap, isolating the helical elements from ground, yet providing conductive paths around its rim interconnecting the helical elements. This antenna is intended primarily as an omnidirectional antenna for receiving circularly polarised signals from sources which may be directly above the antenna, i.e. on its axis, or at smaller angles of elevation down to a few degrees above a plane perpendicular to the axis. It follows that this antenna is particularly suitable for receiving signals from global positioning system (GPS) satellites. Since the antenna is also capable of receiving vertically or horizontally polarised signals, it may be used in other radiocommunication apparatus such as handheld cordless or mobile telephones.
A dielectric-loaded antenna which is particularly suited to portable telephone use is a bifilar helical loop antenna in which two diametrically opposed half turn helical elements form, in conjunction with a conductive sleeve as described above, a twisted loop yielding a radiation pattern which is omnidirectional with the exception of two opposing nulls centred on an axis perpendicular to the plane formed by the four ends of the two helical elements. This antenna is disclosed in our co-pending U.S. patent application Ser. No. 08/664,104 the contents of which form part of the disclosure of the present application by reference. When this loop antenna is appropriately mounted in a mobile telephone handset, the presence of the nulls reduces the level of radiation directed into the user's head during signal transmission. While the antenna gain is superior to many prior mobile telephone handset antennas, it is significantly less than the maximum value above and below a central resonant frequency. It is an object of this invention to provide an antenna of relatively wide bandwidth or capable of operating in two frequency bands.